A cardiac stimulation lead generally includes at least one electrode and a flexible insulated conductor for carrying signals and stimulating pulses between each electrode and a cardiac stimulating device. In the case of a cardiac pacemaker lead, an electrode is passed through the cardiovascular system into a heart chamber, where it is positioned to contact the endocardial wall of the heart. One of various types of fixation devices may be employed to secure an electrode in a fixed position with respect to the cardiac tissue to be stimulated. For example, fins or tines are devices which may be used to passively intertwine with trabeculi within the heart to prevent dislodgement of an electrode. Another common lead fixation strategy employs a sharpened helix, comprising either an electrode itself or a helix adjacent to an electrode, which is affixed to the distal end of an endocardial pacing lead. In this case the helix is rotated by some means from the proximal end of the lead in order to screw the helix into the endocardium and permanently affix the electrode within the heart.
An early method of rotating the helix into heart tissue involved either rotation of the entire lead or rotation of a stylet with a screwdriver tip. Later designs employed fixation helixes that were advanced or retracted into the distal end of the pacing lead to permit passage of the lead through the cardiovascular system without snagging tissue. For example, H. J. Bisping in U.S. Pat. No. 4,106,512, entitled "Transvenously Implantable Lead", issued Aug. 15, 1978, describes a lead in which a helix acts as the electrode and is advanced out of the distal end of the lead by rotation of a coiled conductor within the lead body.
Another approach to rotating a fixation helix out of the distal end of a lead into cardiac tissue employs a screwdriver tip stylet to engage a slot within the distal end of the lead, as taught by R. G. Dutcher in U.S. Pat. No. 4,217,913, entitled "Body-Implantable Lead with Protected, Extendable Tissue Securing Means" issued Aug. 19, 1980. In this patent, the helix serves only to attach the lead to the tissue. A separate ring electrode supplies stimulation pulses. Also, in U.S. Pat. No. 4,570,642, entitled "Endocardial Extendable Screw-In Lead" and issued Feb. 18, 1986, L. M. Kane et al. locate a helix on a member which is slidable within the distal end of the pacing lead. The helix is advanced out of the distal end of the pacing lead by means of a cylindrical stylet which pushes a member located within the distal end of the lead, carrying the fixation helix. The fixation helix is screwed into the tissue by rotation of the entire lead.
For either of these latter two fixation approaches, which involve rotating a helix using a screwdriver tip stylet or rotating a coiled conductor within a lead body, the stylet or coiled conductor does not efficiently transfer torque down the body of the pacing lead. Accordingly, the number of turns of the conductor pin or stylet at the proximal end of the lead does not precisely correspond to the number of rotations of the helix at the distal end of the lead. In U.S. Pat. No. 5,076,285, entitled "Screw-in Lead" issued on Dec. 31, 1991, D. N. Hess et al disclose a fixed helix type pacing lead which provides means for rotationally fixing a stylet located within the lead at both the proximal and the distal ends of the lead. In this case, torque is transmitted down the lead by both the stylet and the coiled conductor within the lead body. This patent also addresses the problem of helix snagging during implantation by provision of a unique helix configuration in which the helix is tapered, and employs a distal, sharpened point which intersects the central axis of the pacing lead.
Active fixation lead assemblies which are known in the prior art present numerous problems and disadvantages. One major problem with prior art lead assemblies is that an implanting physician must position an electrode firmly against heart tissue before fastening the fixation tip in order to achieve a required level of electrical coupling between the electrode and the tissue. Unfortunately, the heart is constantly in motion. Therefore, even when the physician uses an imaging device such as a fluroscope to visualize the relative positions of the lead tip and the heart tissue, the physician still does not know whether the lead tip is firmly and appropriately positioned. Furthermore, the procedure of holding the electrode firmly in place and simultaneously turning the coil is difficult in practice.
Another major problem with prior art lead assemblies is that even though a physician may have implanted a lead and fully extended its coil, the electrode may still not be firmly engaged with the heart tissue, resulting in a poor electrical contact and leading to a high stimulation threshold. Furthermore, if the distal end of the lead is not firmly pressed into the cardiac tissue, the fixation device will not physically engage firmly with the heart tissue and will not electrically couple firmly with the heart tissue. If the electrical coupling is insufficient and a better connection is required, the physician must fully retract the coil and restart the fixation procedure.
An additional difficulty with prior art lead assemblies arises when the lead is firmly positioned against the heart prior to affixing the tip. As a helical coil active fixation tip is rotated, heart tissue is drawn up to the electrode. Unfortunately, prior art lead assemblies provide little indication of when the heart tissue meets the electrode. After the electrode has pressed against the heart, a physician may easily overtorque the helical fixation coil, further embedding the coil in heart tissue and possibly causing tearing of delicate heart tissue.
The present invention provides an improved active fixation lead assembly that specifically overcomes the aforementioned problems and disadvantages of the prior art lead assemblies and generally provides other advantages.
The foregoing problems and disadvantages are overcome and numerous other advantages are provided by an active fixation lead assembly in accordance with the present invention that includes a dual-pitch screw mechanism and a free spinning mechanism. The dual-pitch screw mechanism employs an extendable coupling screw which is firmly attached to a helical fixation coil. The thread pitch of the extendable coupling screw is different from the thread pitch of the helical fixation coil. Generally, the free spinning mechanism allows the fixation coil to rotate freely after the fixation coil has been fully extended. More particularly, the free spinning mechanism of the present invention requires only a relatively low level of torque to operate so that a physician operator can feel the engagement of heart tissue with the coil at the tip of the lead despite the large distance (on the order of three feet) between the lead tip and the stylet knob.
One object of the present invention is to provide an active fixation lead which does not require a physician to position the electrode firmly against the heart tissue prior to advancing the fixation coil. The active fixation lead of the present invention does not require that the electrode be firmly placed against the heart prior to affixing the coil. The electrode may be placed in the general vicinity of a desired part of the heart and the coil may be rotated to extend the coil from the lead. After the fixation coil is fully extended, it will continue to rotate freely, drawing the heart tissue up snugly against the distal end of the lead assembly.
A second object of the present invention is to provide an active fixation lead which continues to draw heart tissue toward the electrode even after the fixation coil is fully extended. The active fixation lead of the present invention employs a dual-pitch screw in which the thread pitch of a distal coil portion of the screw is greater than the thread pitch of a proximal coupling screw portion so that the distal coil portion will move through tissue faster per revolution than the proximal screw advances relative to the lead, thereby embracing the tissue more tightly against the electrode housing and improving the electrical connection between the tissue and the electrode. Furthermore, the active fixation lead of the present invention includes a free spinning mechanism. After the fixation coil is fully extended, it will continue to rotate freely, drawing the heart tissue up snugly against the distal end of the lead assembly. Thus, the heart tissue is firmly held by a fixation coil, snugly against the distal end of the electrode. An increased intimacy between the heart tissue and the cardiac stimulating and sensing electrodes increases the surface area of the electrode-tissue interface and reduces the microscopic motion between the electrode and the tissue, thereby reducing irritation of the tissue and improving pacing thresholds. The lead is easier to implant because the lead fixation system does not require that the implanting physician maintain pressure on the lead prior to coil extension.
A third object of the present invention is to provide an active fixation lead in which a tactile feed back signal is given to the implanting physician to cease rotation of the helical fixation coil after the heart tissue is snugly drawn against the electrode. The free spinning mechanism of the active fixation lead of the present invention facilitates such signalling. As the helical coil rotates, the heart tissue is drawn up toward the lead assembly. When the heart tissue meets the lead assembly, the tissue exerts an immediate and positive resistance to further rotation of a stylet knob, providing a significant tactical feedback to the physician indicating that further rotation of the stylet knob is unnecessary and might be injurious to heart tissue. Thus, the active fixation lead helps to prevent accidental advancement of the fixation coil due to inadvertent torque being applied to a stylet during advancement.
An additional object of the present invention is to provide rotational force to a helical coil lead tip using a screwdriver tip stylet which more efficiently transfers torque down the body of the pacing lead. The dual-pitch screw of the present invention is provided with a thread pitch on its distal coil portion which is greater than the thread pitch on its proximal screw portion so that the distal coil portion will move through tissue faster per revolution than the proximal screw portion advances relative to the lead, thereby increasing the efficiency of the fixation procedure by embracing the tissue more tightly against the electrode housing and improving the electrical connection between the tissue and the electrode.
A further object of the present invention is to reduce the size of an electrode housing which holds the screw mechanism of an active helical coil fixation lead. Prior art electrode arrangements have employed a machined or molded nut to hold a coupling screw. In the present invention, a small ball bearing replaces the larger nut.
A still further object of the present invention is to provide a seal located between heart tissue and the threaded mechanism. In prior art active fixation leads, the threaded mechanism was exposed to blood.
Further objects and advantages of the invention will become apparent as the following description proceeds.